1. Field of the Invention
The present invention relates to an optical isolator, and more particularly to an optical isolator that can be preferably used for optical isolation in the wavelength range of 320 to 800 nm.
2. Description of the Related Art
In recent years, with advancement of laser processing machines, magneto-optic devices utilizing mutual interaction of light and magnetism have attracted attention.
As one of such devices, there is an optical isolator (see, e.g., Non-patent Literature 1 and others).
This device is configured to suppress a phenomenon that, when light oscillated from a laser light source is reflected by an optical system provided along the path and thereby returns to the light source, the light oscillated from the laser beam is disturbed to make an unstable oscillation state.
Therefore, for utilizing this operation, the optical isolator is utilized with arranging between the laser light source and an optical component.
This optical isolator mainly has three components, i.e., a Faraday rotator, a polarizer arranged on a light incidence side of the Faraday rotator, and an analyzer arranged on a light exit side of the Faraday rotator (the polarizer placed on an opposite side with respect to the Faraday rotator is also referred to as the analyzer).
Further, the optical isolator utilizes a so-called a Faraday effect, i.e., properties that a polarization plane is rotated in the Faraday rotator when light incidents the Faraday rotator in a state that a magnetic field is applied to the Faraday rotator in parallel to a traveling direction of the light.
That is, light having the same polarization plane as that of the polarizer in incident light passes through the polarizer and incidents the Faraday rotator. This light is rotated +45° with respect to the traveling direction of the light in the Faraday rotator and exits.
On the other hand, in regard to return light that incidents the Faraday rotator from an opposite direction of the incidence direction, light component that has the same polarization plane as that of the analyzer alone passes through the analyzer when first passing through the analyzer, and the light then incidents the Faraday rotator.
Furthermore, in the Faraday rotator, since the polarization plane of the return light is further rotated +45° from the first +45°, the polarization plane forming a right angle, i.e., +90° with respect to the polarizer is provided, and the return light cannot pass through the polarizer.
The optical isolator utilizes this phenomenon to prevent the return light from being produced.    Non-patent Literature 1: “Applied Optical Electronics Hand Book” edited by Applied Optical Electronic Hand Book Editorial Committee, pp. 77-78, Shokodo Co., Ltd.
Here, a material used for the Faraday rotator of the above-described optical isolator must have the high Faraday effect and a high transmission factor with respect to light having a wavelength utilized.
That is, for recently demanded miniaturization of the optical isolator, a thickness of the Faraday rotator need to be reduced, and therefore using a crystal having the high Faraday effect is the most realistic solution.
Moreover, attenuating the light in a traveling direction has a problem, and hence a higher transmission factor of the light is more advantageous.